Sofa back



FIG. 1 is a front perspective view of an exemplary sofa back;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a left side view thereof; and,

FIG. 5 is a right side view thereof.

The dashed broken line showing in the Figures is for illustrativepurposes only and forms no part of the claimed design.

CLAIM The ornamental design for a sofa back, as shown and described.